Wireless communication systems, for example cellular telephony or private mobile radio communication systems, typically provide for radio telecommunication links to be arranged between a plurality of base transceiver stations (BTS), referred to as Node Bs with regard to 3rd generation (3G) cellular systems such as the universal mobile telecommunication system (UMTS) systems, and a plurality of subscriber units, often referred to as user equipment (UE) in UMTS systems. The communication link from a Node B to a UE is generally referred to as a down-link communication channel. Conversely, the communication link from a UE to a Node B is generally referred to as an up-link communication channel.
In such wireless communication systems, techniques for communicating information simultaneously exist, where communication resources are shared by a number of users. Such techniques of sharing resources are termed multiple access techniques. A number of multiple access techniques exist, whereby a finite communication resource such as frequency and/or time is divided into any number of physical parameters, such as frequency channels or time periods (slots/frames, etc.).
The present invention will be described with respect to a 3rd generation partnership project (3GPP) communication system based on the UMTS standard. 3 G communication systems employ a Code Division Multiple Access (CDMA) technique, whereby substantially all communications are able to use a selection from all of the respective frequencies in all of the available time periods. In effect, the resource is shared by allocating each communication a particular code in order to differentiate desired signals from undesired signals. This is often referred to as spread spectrum signaling. Some communication resources (often termed channels) are used for carrying data (traffic) and other channels are used for transferring control information, such as call paging, between the Node B and the UEs.
Two categories of spread spectrum communications are direct sequence spread spectrum (DSSS) and frequency hopping spread spectrum (FHSS). In the case of a DSSS communication system, for example, multiplying the data content of the signal with a wide-band pseudo-random code can most easily spread the spectrum of a signal. It is essential that the receiver precisely know the spreading signal, so that the receiver is able to ‘de-spread’ the signal, in order to recover its original content. A cellular communication system using DSSS is commonly known as a Direct Sequence Code Division Multiple Access (DS-CDMA) system, one example of which is defined in the TIA-EAI standard IS-95. Thus, individual users in the system use the same radio frequencies (RF) and time slots, but they are distinguishable from each other by the use of individual spreading codes. Hence, multiple communications channels are allocated using a number of spreading codes within a portion of the radio spectrum. Each code is uniquely assigned to a UE, except for common channels.
In order to decode the correct spreading code, a special form of signal receivers is used, often referred to as RAKE receivers. RAKE receivers employ equalization functions to equalize signals appearing at different time instances, when transmitted on different frequency channels. Such signals may also suffer from multipath and other wireless propagation effects that need to be compensated for by the receiver's equalizer function.
However, in the art of CDMA systems, it is known that RAKE receivers do not provide an adequate performance in the presence of severe interference, such as multiple access interference (MAI) or inter-symbol (ISI) interference. Hence, there is a constant demand for enhancing a CDMA receiver's performance, particularly in the context of a UE receiver downlink performance, by designing improved spread spectrum receivers.
One of the most promising candidates to replace the well-known RAKE receiver is a linear chip equalizer, which is designed especially for the downlink channel. The performance of chip equalizers is evaluated by 3GPP, and most probably will be a basis for performance requirements of the future release of the high-speed data packet access (HSPDA) standard within 3G systems.
A. Klein describes an equalizer for a CDMA downlink channel using linear zero-forcing (ZF) and minimum-mean-squared-error (MMSE) techniques, in the paper “Data detection algorithms specially designed for the downlink of CDMA mobile radio systems”, VTC'97. In this paper, the equalization problem is solved on a data ‘symbol’ level, in a sense that the optimization of the mean-squared error is performed on the despreaded user symbol.
Another approach to equalization is to consider the composite chip sequence, which is the sum of spreaded signals of all users in a cell. A processor in the receiver unit then solves the ZF and MMSE problems on a chip level, rather than at a symbol level. An example of such an approach is described in I. Ghauri and D. T. M. Slock's paper: “Linear receivers for the DS-CDMA downlink exploiting orthogonality of spreading sequences”.
A relatively simple solution to the aforementioned problem can be obtained if the composite chip sequence is assumed to be independent and identically distributed (iid), as described by T. P. Krauss, M. D. Zoltowski and G. Leus, in their paper “Simple MMSE equalizers for CDMA downlink to restore chip sequence: comparison to zero-forcing and RAKE”. In this case, no spreading/scrambling code information is needed and the co-efficients of the linear equalizer are found using the channel response and the noise variance only.
Since in reality the channel response is not known by the receiver, the most common approach is to use a training sequence for channel estimation and computation of the equalizer taps. In the 3G cellular WCDMA standard, a code-multiplexed ‘pilot’ signal is provided. The pilot signal is used for the purpose of providing a training sequence that undergoes substantially the same propagation conditions as the primary data transmitted on the traffic channel. The decoded pilot signal is then used for channel estimation and computation of the equalizer taps (filter co-efficients). The pilot signal may be employed in either a block-based or adaptive equalizer configuration, for example as described in the paper by: F. Petre, M. Moonen, M. Engels, B. Gyselinckx and H. De Man, titled “Pilot-aided adaptive chip equalizer receiver for interference suppression in DS-CDMA forward link”, published in VTC 2000.
It is known that the performance of an equalizer relying on the pilot signal technique can be improved by a semi-blind approach, as described in the paper by: F. Petre, G. Leus, M. Engels, M. Moonen and H. De Man, titled “Semi-blind space-time chip equalizer receivers for WCDMA forward link with code-multiplexed pilot”, published in ICASSP'01. This technique is based on optimization using the equalizer's filter coefficients and transmitted data. The clear drawback in this case is higher complexity, since a semi-blind equalizer requires auto-correlation and cross-correlation properties of the user codes. These problems are compounded in the case of a long scrambling code, as the codes change from symbol-to-symbol.
Another important path in the evolution of existing cellular communication systems is the introduction of an adaptive modulation and coding (AMC) technology. The idea of AMC systems is to adapt the modulation and the coding rate, and consequently to vary the data rate, according to the prevailing channel conditions. Typically, such systems employ a special downlink control channel signaling comprising the modulation type, coding rate and other parameters needed for decoding of the transmitted data. It is known that for ‘poor’ channel conditions the data rate is reduced, whereas for a ‘good channels’ the data rate is increased up to the maximum extent, at which it can still be copied by a UE. Thus, by employing AMC the average data throughput of the communication system is increased.
A need therefore arises for a CDMA receiver to provide increased data throughput, particularly with regard to adaptive modulation and coding WCDMA communication systems.